This invention relates generally to apparatus for inserting prefabricated vertical (PV) drains (sometimes referred to as wick or band drains) into the earth, and more specifically to an improved apparatus for inserting PV drains into dense or hard soil layers.
One well known technique for preparing soil that has a high moisture content, such as wet clay, for example, is to drive into the soil a drainage element that penetrates deep into the soil with the top end of the drainage element maintained above the surface of the soil. The PV drain is formed of any suitable material which is water permeable, or perforated to be water permeable, so that the water in the soil can penetrate the walls of the drain and flow upwardly therein to the surface of the soil as a result of water pressures in the soil beneath the surface. It is common practice in such situations to increase these inherent water pressures in the soil by placing a layer of earth on top of the wet soil so that the weight thereof will assist in forcing the water into and upwardly through the PV drains, where the water can be readily disbursed.
The PV drain is generally elongated and flexible and it is carried into the ground by utilizing a rigid insertion tube or mandrel formed of a suitable metal. This insertion tube, together with the drain, is driven downwardly into the earth to the desired depth and then the insertion tube is pulled out of the soil thereby leaving the PV drain. The drains are inserted at regular predetermined intervals in the earth, depending upon the soil conditions and moisture content.
This rigid insertion tube or mandrel which carries the elongated flexible PV drain therein, is adapted for vertical movement within a mast. The insertion tube is generally driven downwardly into the earth and retracted therefrom with a drive mechanism. This drive mechanism is preferably positioned at the bottom or lower end of the mast, and by way of example, may consist of a pair of friction rollers, or even more preferably, a gear drive mechanism which is positioned at the lower end of the mast as disclosed in U.S. Pat. No. 5,213,449 for APPARATUS FOR INSERTING WICK DRAINS INTO THE EARTH issued to Morris, which is incorporated herein by reference. Normally a vehicle is provided which supports the mast at its base and is further adapted to raise and lower the mast to and from vertical and to otherwise manipulate and transport the mast.
The present invention pertains to an improvement to the gear drive apparatus disclosed in the Morris patent. The Morris patent discloses a number of prior art devices for inserting wick drains into the earth. These include a vibratory driver that engages the top of the insertion tube or mandrel, hydraulic cylinders and cables that engage the tube at its upper end, chain drives that also engage the tube at its upper end, etc. It is also pointed out that it is known to utilize vibratory means in combination with cable or chain drives, commonly referred to as vibro/static machines.
In general most of these prior art arrangements engage and drive the insertion tube at its top end, requiring a relatively heavy mast and boom arrangement to support the insertion tube or mandrel and the drive mechanism. This not only increases the weight of the apparatus, but also increases the cost of fabrication as well as the maintenance.
This prior art friction roller arrangement overcomes the problem of engaging the insertion tube at its upper end, but suffers from the tendency of the friction rollers to slip when the mandrel or insertion tube is covered with wet, slippery soil material which adheres to the mandrel. The Morris patent overcomes this problem by utilizing a drive gear to positively engage a flange or fin which is attached to and coextends with the mandrel. This flange contains rack gear mesh openings spaced along its length, which the teeth of the drive gear engage. This mechanism is similar to a rack and pinion arrangement.
From a geotechnical standpoint, it is preferable to install vertical drains without the use of vibration, since such vibration can remold the soil in close proximity with the mandrel, resulting in loss of strength and decrease in permeability. Lower permeability of the soil in this region impedes the flow of water into the drain, requiring longer surcharge periods. However, vibration greatly enhances the ability of the apparatus to penetrate, and it is often necessary to penetrate through dense or hard soil layers to reach an underlying soft soil layer. These layers are often so hard that it is not possible to penetrate them without the use of a vibratory mechanism. The combination machines (vibro/static machines) are very useful in these cases, since the vibration can be turned on only during penetration through the hard layers. Further, vibrating the mandrel induces very high stresses, and fatigue of the mandrel material becomes a problem.
The present invention discloses means to add vibratory capability to the gear drive apparatus disclosed in the Morris patent. The present invention permits the apparatus of the type disclosed in the Morris patent to operate without vibration to the supporting equipment and the vibrating shock imparted to the mandrel is isolated from the motor and planetary gear box and the remainder of the machine. This permits application of vibrations to the mandrel either intermittently or constantly as required and greatly extends the usefulness and life of the machine.